I always say that the front wing of a Formula 1 car is the single most important aerodynamic component because it has two roles. As well as producing downforce in its own right, it dictates the quality of the airflow to all the aerodynamic surfaces following it. Coming a close second to the front wing is the area inside the rear tyres known as the 'Coke bottle'. Since Alan Jenkins came up with the initial concept for McLaren in the mid-1980s, this is the area that has made single-seater racing cars much more open-wheeled.
This area, and how it functions, is critical to the amount of airflow that is displaced either inside or outside of the rear tyre. If the majority of the flow has to go around the outside of the tyre the drag levels will be increased. In effect this makes the car wider the further rearward you go, which in turn will reduce efficiency of the complete car.
However, persuading more air to flow to the inside is not as simple as making as big a gap as possible by narrowing the bodywork. If the profile of the sidepods is not correct it is very easy to get airflow separation on the inner surface when the car is mid-corner. If this happens it will affect rear downforce and cause the car to snap into oversteer.
Optimising this area is all about understanding the airflow regime coming around the undercut area of the front of the sidepods and any influence that the front wheels will have on that airflow - especially when the steering angle changes. Add to this the effect the hot-blown exhaust exits have on this airflow and it is easy to see why some teams have taken most of this season to get a grip on it.